This invention relates to systems for controlling operation of a multi-speed circulation blower in a heating and cooling apparatus, and particularly to such systems wherein the number of times a high limit switch has opened during a single call for heat determines the blower speed.
In gas-fired, warm-air furnaces, it is desirable to prevent the temperature in the plenum from exceeding a certain value so as to protect the furnace and its environment from the effects of excessively high temperatures. Accordingly, a high limit switch is typically located in the plenum. The high limit switch opens when the sensed plenum temperature rises to a certain value. If the thermostat is still calling for heat, the opening of the switch breaks the electrical power to the gas valve so that the gas flame is extinguished. It is desirable that the circulation blower in the furnace be energized under this condition so that the excessive heat is removed from the plenum. Therefore, when the high limit switch opens, the blower is energized, if it is not already energized. In the prior art, this function has been provided either directly by the high limit switch by utilizing a high limit switch having a set of normally-open contacts which close on temperature rise and complete an electrical circuit directly to the circulator blower, or indirectly by the high limit switch acting through a microcomputer-based furnace control.
In the microcomputer-based furnace control arrangement, there are usually two or more outputs from the microcomputer for controlling the circulation blower. During the heating mode, one output effects energizing of a heat speed winding of the blower motor, and during the cooling mode, another output effects energizing of a cool speed winding of the motor. In response to an opening of the high limit switch, which would occur only during the heating mode, the output that is in control of the blower is the output which effects energizing of the heat speed winding.
There are several possible reasons why the high limit switch may open. One reason is that the air filter in the circulation path may be excessively dirty so that the filter does not permit enough air to be drawn out of the plenum, with the heat speed velocity, to cool it sufficiently. Another reason is that the furnace control may be defective. For example, a relay or a relay drive circuit in the furnace control may be defective so that the heat speed winding of the circulation blower motor is not energized when the controlling circuitry indicates it should be energized.
If the high limit opened due to a dirty filter, enough heat should be removed from the plenum and distributed to the conditioned space by the circulating air, even with a dirty filter, so that the thermostat should become satisfied before the high limit has a chance to close again. However, if the high limit opened due to the heat speed winding of the circulation blower motor not being energized when the controlling circuitry indicates it should be energized, the response to the opening of the high limit of re-energizing the heat speed winding of the blower motor has no effect on blower operation. That is to say, the blower will not operate, no heat will be circulated to the conditioned space, and the plenum will again be heated, causing the high limit to open again. Such operation could continue until the control locks out, terminating all furnace activity for at least some period of time. If the conditioned space is occupied, such lack of heat would be observed and corrective action could be taken, such as calling a service man. However, if the conditioned space is unoccupied, such inadequate heating would go unnoticed until the occupants returned. Depending on the temperature of the conditioned space, such inadequate heating could result in, for example, frozen water pipes. To prevent such inadequate heating, it is desirable that some means be provided for circulating warm air if the problem is caused by defective heat speed operation of the blower.